The present invention relates generally to trace gas detection and more specifically to cavity-based spectroscopy systems and methods for measuring one or more trace gases.
In cavity enhanced optical absorption spectroscopy systems and methods, radiation of a laser is directed into a resonant optical cavity, and the optical intensity inside the cavity is observed. The optical frequency of the laser can be periodically scanned. If it is assumed for clarity that the laser linewidth is much smaller than the cavity resonance width, at the moment when the laser light frequency coincides with a cavity mode transmission peak the optical intensity inside the resonant optical cavity reflects total cavity loss, and the total cavity loss can be quantitatively determined provided that the incident intensity and cavity parameters are known. The total cavity loss is a sum of the cavity mirror losses and losses caused by absorption of a gas mixture present in the cavity. The lower the cavity mirror losses, or equivalently, the higher each mirror's reflectivity—the smaller the absorption of the intra-cavity gas mixture that can be detected. With very high reflectivity mirrors, the laser linewidth will become too large compared to the cavity resonance width, thus limiting achievable enhancement of the gas mixture absorption by the cavity. This can be helped by narrowing the laser linewidth using optical feedback from the cavity and a laser that is sensitive or responsive to optical feedback from the cavity. With such a feedback sensitive laser, during the scan, as the frequency of the laser light approaches the frequency of one of the cavity modes, the laser locks to that mode, i.e., the laser linewidth becomes much smaller than the resonance mode width, and that regardless whether the frequency scan range of the unlocked laser may be large, in a locked condition the optical frequency of the laser will change only within the resonance peak. As the laser frequency scan continues, the laser will lose the lock to the current cavity mode and relock to the next cavity mode that it approaches. Due to the optical feedback effect, the laser optical frequency during the scan will essentially take the number of discrete values corresponding to the peaks of the cavity mode resonances that are equidistant in optical frequency. A discrete absorption spectrum of the analyzed gas can thus be obtained by sequential coupling to the entire set of the cavity modes within the scan range, and the trace gas concentration can be derived from the absorption spectrum. This sub-family of cavity-based spectroscopy systems and methods that uses optical feedback will be referred to as optical feedback cavity enhanced absorption spectroscopy (OF CEAS).
In OF CEAS, the strength of the optical feedback from the resonance cavity to the laser should be within certain limits, otherwise it may not be possible to provide reproducible scan-to-scan mode coupling as the laser scans. Previous OF CEAS systems strive to optimize optical power coupled into an external optical cavity in a manner that enhances cavity feedback. Such coupling requires weak optical feedback to the laser such that the resonant enhancement does not last more than one Free-Spectral-Range (FSR). In known OF CEAS systems and methods, complex optical components are used to control the feedback. Such components include Faraday isolators, variable optical attenuators, or polarization rotators. One example is provided in U.S. Pat. No. 7,450,240, which uses a settable attenuator element such as a Faraday Isolator to control the amount of light fed back towards the laser. The settable attenuator actively reduces the feedback light emerging from the cavity, and incident upon the laser, to a desirable level. This does provide a way to control the feedback but at the expense of added complexity, cost, and added sources of noise and instability. For example, adverse interference effects, temperature drifts and aging drifts may result from these components in the system. Achieving high stability and high reproducibility of the optical absorption measurements becomes a major problem in such systems.
U.S. Pat. No. 8,659,758 avoids the complications in the system design of U.S. Pat. No. 7,450,240 by specifying that one of the mirrors of the external cavity (the coupling mirror) have a low transmission level. This is a much simpler approach compared to U.S. Pat. No. 7,450,240 and avoids the disadvantages associated therein. However, the approach in U.S. Pat. No. 8,659,758 presents challenges of its own. For example, the low transmission requirement is difficult to achieve batch-to-batch, and when other components of the design change, such as the laser, the amount of transmission of the coupling mirror needs to be changed accordingly. These challenges may present difficulties in a production environment.